In the recovery of oil from oil-containing formations, it is usually possible to recover only minor portions of the original oil-in-place by so-called primary recovery methods which utilize only natural forces. To increase the recovery of oil a variety of supplementary recovery techniques are employed. These techniques include waterflooding, miscible flooding, and thermal recovery.
A problem that arises in various flooding processes is that different strata or zones in the reservoir often possess different permeabilities. Thus, displacing fluids enter high permeability or "thief" zones in preference to zones of lower permeability. Significant quantities of oil may be left in zones of lower permeability. To circumvent this difficulty the technique of profile control is applied to plug the high permeability zones with polymeric gels and thus divert the displacing fluid into the underswept low permeability, oil rich zones. Among the polymers examined for improving waterflood conformance are metal cross-linked polysaccharides, metal cross-linked polyacrylamides, and organic-crosslinked polyacrylamides.
Polymeric gels are disclosed in several U.S. patents. Among these is U.S. Pat. No. 4,157,322 which issued to Colegrove on June 5, 1979. This gel is formed from water, a polysaccharide polymer, an acid generating salt and a melamine resin. A polymeric gel is disclosed in U.S. Pat. No. 4,658,898 which issued to Paul et al. on Apr. 21, 1987. This patent discloses an aqueous solution of heteropolysaccharide S-130 combined with cations of basic organic compounds which cations contained at least two positively charged centers. U.S. Pat. No. 4,716,966, issued to Shu on Jan. 5, 1988, discloses a gel formed by amino resins such as melamine formaldehyde which modify biopolymers in combination with transitional metal ions. These patents are hereby incorporated by reference herein.
Basic to the problem of diverting displacing fluid with polymeric gels is the necessity of placing the polymer where it is needed, i.e. in the high permeability zone. This is possible when xanthan biopolymers are cross-linked with metal ions such as Cr.sup.+3 above ground to give gels. These gels are shear stable and shear thinning. They can be injected into the formation where they then reheal. Due to the gel's reological properties, they will of necessity go into high permeability zones. However, many other gel systems are formed in-situ. One system disclosed in U.S. Pat. No. 3,557,562 contains acrylamide monomer, methylene-bis-acrylamide as an organic cross-linker, and a free radical initiator. This system undergoes polymerization in the formation to give a polyacrylamide cross-linked with methylene-bis-acrylamide. However, the viscosity of the solution when injected is like that of water. Unless mechanical isolation is used, these solutions are quite capable of penetrating low permeability, oil bearing zones. Another form of in-situ gelation involves the injection of polyacrylamide containing chromium in the form of chromate. A reducing agent such as thiourea or sodium thiosulfate is also injected to reduce the chromate in-situ to Cr.sup.+3, a species capable of cross-linking hydrolyzed polyacrylamide. Even though the polyacrylamide solution has a viscosity greater than water, it is not capable of showing the selectivity that a gel can. Thus, polyacrylamides cross-linked with chromium in-situ can also go into low permeability zones. It is not useful to cross-link polyacrylamides above ground and inject them as gels, because polyacrylamide gels undergo shear degradation. There are very few gels that are selective and thermally stable.
In addition to the creation of "thief" zones during a waterflooding recovery technique, steam flooding or steam stimulation processes create a unique situation in a formation which is gravity override due to the steam's low density. Because of the steam's low density, the sweep path of the steam is therefore biased towards the top of the payzone. Thus, the area invaded by the override steam may or may not be of low permeability. Although steam preferentially enters a high permeability or thief zone, the high temperature of the steam will also remove hydrocarbonaceous fluids from portions of low permeability zones. As the distance increases from steam entry into the formation, the temperature will decrease. As long as the temperature is hot enough hydrocarbonaceous fluids will be removed from the "thief" zone as well as portions of low permeability zones.
When steam has broken through to a production well during a steamflood, a heated portion of the formation will communicate with an injector well. This heated portion may include portions of both high and low permeability steam override zones. For this reason, a gel system which can selectively enter a high permeability "thief" zone may not enter a low permeability zone. Thus, a size selective gel may not prevent steam from entering into a heated low permeability zone which has been depleted of hydrocarbonaceous fluids.
Therefore, what is needed is a method whereby a gel forms in-situ in a steam overswept zone of a formation only when said zone has been previously heated during a steamflood enhanced oil recovery process regardless of its permeability.